Electrography pertains to forming and utilizing latent electrostatic charge patterns to record and reproduce patterns in visible form. This field was pioneered by Chester F. Carlson when he disclosed in U.S. Pat. No. 2,297,681 the basic techniques of one major sector of the field, referred to as electrophotography. In the most commonly practiced form of electrophotography a photoconductive element is first given a uniform electrostatic charge over its entire surface. The element is then exposed to an image of actinic electromagnetic radiation such as light which selectively dissipates the charge in illuminated areas of the photoconductive element, while charge in the non-illuminated areas is retained, thus forming a latent electrostatic image.
The latent electrostatic image may then be developed or made visible by the deposition of finely divided electroscopic toner on the surface of the photoconductive element, as a result of which the toner conforms to the pattern of the latent electrostatic image. The visible image may be utilized in a number of diverse ways. For example, the image may be fixed in place on the photoconductive element or transferred to a second surface such as a sheet of paper and fixed in place. Likewise, the electrostatic charge pattern can be transferred to a second surface and developed there.
Another broad general branch of electrography is generally considered distinct from the electrophotographic branch in that it does not employ a photoconductive element and electromagnetic radiation to form its latent electrostatic image. This branch of electrography may generally be divided into two broad sectors which are generally referred to as xeroprinting and electrostatic or TESI recording (an acronym for the phrase "Transfer of Electrostatic Images").
Xeroprinting is considered to be the electrostatic analog of ordinary printing. The xeroprinting process, which is more fully described in U.S. Pat. No. 2,576,047 to Schaffert, employs a xeroprinting plate made up of a pattern of insulating material which is generally on a conductive backing so that when the xeroprinting plate is charged, as with a corona discharge electrode, an electrostatic charge pattern is retained only on the patterned insulating sections of the plate. This electrostatic image may then be developed with the same developing materials and techniques employed in developing electrophotographic images.
In electrostatic or TESI recording, the electrostatic charge patterns conforming to the desired reproduction are formed on a uniform insulating layer by means of an electrical discharge between two or more electrodes on opposite sides of the insulating medium. By controlling the shapes, combinations and numbers of electrodes employed, charge patterns of almost any shape may be formed on the insulating medium. Again, image development is by the same techniques as in electrophotography.
The common feature of all of these electrographic systems is that they employ the lines of force from an electric field to control the deposition of finely divided toner particles on a surface, thus forming an image with the toner particles. Although these systems are generally used for black and white reproduction, they are capable of forming images in either a single color or a combination of colors.
When a full color electrographic system is desired it is generally based on trichromatic color synthesis of either the additive or subtractive color formation types. Thus, when electrographic systems are operated in full color, toner or developing particles of at least three different colors must be employed to synthesize a desired color. As a rule, at least three color separation images are formed and combined in register with each other to form a colored reproduction of the original. Thus, in color xeroprinting or electrostatic recording at least three different latent electrostatic images must be formed, developed with different colored toners and combined to form the final image. The same is true of color electrophotography where at least three latent electrostatic images are formed by exposing a photoconductive element to different optical color separation images and developing each of these latent electrostatic images with a different colored toner, after which the three toner images are combined to form the final image.
In the systems described hereinbefore, the combination of the three color toner images may be provided on a receiver sheet such as paper, film, plastic or glass, to which the images are permanently fixed. The most common technique for fixing these toner images to the receiver sheet is by employing a thermoplastic polymeric toner which includes a colorant and heat fusing the toner images to the receiver sheet. The images may also be fixed by other techniques known in the art, such as subjecting them to a solvent vapor or by the use of a lacquer overcoat.
A toner employed in an electrographic color process must possess certain characteristics. For instance, the toner must be of the proper hue. In a typical substractive trichromatic process, combinations of magenta, cyan and yellow toners are used to produce images of their complementary colors: green, red and blue. Ideally, the absorption bands of the colored toners would be narrow, without extensive overlap, in order to allow production of saturated color images. Narrow absorption bands are especially desirable when a half-tone process is employed because half-tone imaging tends to broaden the reflection spectra due to scattering from the support to which the toner image is fixed, e.g. paper.
Further, there are certain other characteristics which are highly desirable for electrographic colorants. These characteristics include high extinction coefficient, stability to light, compatibility with the polymeric binder and transparency. A high extinction coefficient allows the use of less dye to obtain the desired color density. Light stability is important since fading can render the color image aesthetically undesirable. For example, Japanese Patent Application Publication (Kokai) No. 57-130044 discloses a toner comprising C.I. Solvent Red 49 (rhodamine free base) and C.I. Solvent Red 52 in a binder resin. Tests have shown (see comparison Example B, hereinafter) that toner of this type exhibits poor light stability; fading 30% in the green region of absorption spectra after 7 days exposure to high intensity daylight (HID).
Compatibility of the toner colorant and the polymeric binder is also important. Good dispersion of the colorant is essential to minimize unwanted light scatter since such scatter leads to broadening the reflectance spectrum of the colorant and to desaturation of the resultant hue. In addition, good dispersion results in maximum utilization of colorant which can be expressed as optical density obtained per gram of colorant employed. Adequate dispersion of insoluble particulate colorants is best achieved in binders with which they are compatible. The ultimate in compatibility is achieved with colorants which form solutions in the binder. Thus, colorants should be chosen which can be finely divided or dissolved in the toner binder to give maximum color saturation and colorant utilization.
A high degree of transparency of toner colorants avoids image degradation due to light scatter. Generally, prior art color electrographic systems operate by laying the color separation images on top of one another. In this system the toner images are superimposed and the toners must be sufficiently transparent so that no one of the three toner colors will scatter the light from the other different colored toner images. High color saturation and brightness are needed to satisfy the colorimetric requirements for three color synthesis of natural color images. The requirements of high transparency and good color saturation are extremely difficult to satisfy.
Japanese Patent Application Publication (Kokai) No. 52-80839 describes a toner comprising a magenta pigment obtained by treating a dye with phosphorus tungsten molybdate to form a so-called "lake pigment". The dye used to form the lake pigment has the formula: ##STR1## where each of R.sub.1-7 is H or lower alkyl and A.sup.- is an anion. There is no indication in the Japanese Patent Application Publication that such a dye (not the lake pigment) could be used to form a magenta toner, and especially the magenta toner having the superior combination of characteristics described hereinafter.